1. Field of the Invention
The present invention relates to a refrigerant compressor, and more particularly, to a slant plate type refrigerant compressor, such as a wobble plate type refrigerant compressor, with a variable displacement mechanism suitable for use in an automotive air conditioning system.
2. Description of the Prior Art
A wobble plate type refrigerant compressor with a variable displacement mechanism as illustrated in FIG. 1 is disclosed in U.S. Pat. No. 4,960,367 to Terauchi. For purposes of explanation only, the left side of the Figure will be referenced as the forward end or front and the right side of the Figure will be reference as the rearward end.
Compressor 10 includes cylindrical housing assembly 20 including cylinder block 21, front end plate 23 at one end of cylinder block 21, crank chamber 22 formed between cylinder block 21 and front end plate 23, and rear end plate 24 attached to the other end of cylinder block 21. Front end plate 23 is mounted on cylinder block 21 forward of crank chamber 22 by a plurality of bolts 101. Rear end plate 24 is mounted on cylinder block 21 at its opposite end by a plurality of bolts 102. Valve plate 25 is located between rear end plate 24 and cylinder block 21. Opening 231 is centrally formed in front end plate 23 for supporting drive shaft 26 by bearing 30 disposed in the opening 231. The inner end portion of drive shaft 26 is rotatably supported by bearing 31 disposed within central bore 210 of cylinder block 21. Bore 210, which extends to a rearward end surface of cylinder block 21, contains valve control mechanism 19' as discussed below.
Cam rotor 40 is fixed on drive shaft 26 by pin member 261 and rotates with drive shaft 26. Thrust needle bearing 32 is disposed between the inner end surface of front end plate 23 and the adjacent axial end surface of cam rotor 40. Cam rotor 40 includes arm 41 having pin member 42 extending therefrom. Slant plate 50 is adjacent cam rotor 40 and includes opening 53 through which drive shaft 26 passes. Slant plate 50 includes arm 51 having slot 52. Cam rotor 40 and slant plate 50 are connected by pin member 42, which is inserted in slot 52 to create a hinged joint. Pin member 42 is slidable within slot 52 to allow adjustment of the angular position of slant plate 50 with respect to a plane perpendicular to the longitudinal axis of drive shaft 26.
Wobble plate 60 is rotatably mounted on slant plate 50 through bearings 61 and 62. Fork shaped slider 63 is attached to the outer peripheral end of wobble plate 60 and is slidably mounted on sliding rail 64 held between front end plate 23 and cylinder block 21. Fork shaped slider 63 prevents rotation of wobble plate 60 so that wobble plate 60 nutates along rail 64 when cam rotor 40 rotates. Cylinder block 21 includes a plurality of peripherally located cylinder chambers 70 in which pistons 71 reciprocate. Each piston 71 is connected to wobble plate 60 by a corresponding connecting rod 72.
Rear end plate 24 includes peripherally located annular suction chamber 241 and centrally located discharge chamber 251. Valve plate 25 is located between cylinder block 21 and rear end plate 24 and includes a plurality of valved suction ports 242 linking suction chamber 241 with respective cylinders 70. Valve plate 25 also includes a plurality of valved discharge ports 252 linking discharge chamber 251 with respective cylinders 70. Suction ports 242 and discharge ports 252 are provided with suitable reed valves as described in U.S. Pat. No. 4,001,029 to Shimizu.
Suction chamber 241 includes inlet portion 241a which is connected to an evaporator of the external cooling circuit (not shown). Discharge chamber 251 is provided with outlet portion 251a connected to a condenser of the cooling circuit (not shown). Gaskets 27 and 28 are located between cylinder block 21 and the front surface of valve plate 25, and the rear surface of valve plate 25 and rear end plate 24 respectively, to seal the mating surfaces of cylinder block 21, valve plate 25 and rear end plate 24.
With reference to FIG. 2, valve control mechanism 19' includes cup-shaped casing member 191 defining valve chamber 192 therewithin. O-ring 19a is disposed between an outer surface of casing member 191 and an inner surface of bore 210 to seal the mating surfaces of casing member 191 and cylinder block 21. A plurality of holes 19b are formed at the closed end (to the left in FIGS. 1 and 2) of casing member 191 to expose valve chamber 192 to the crank chamber pressure through gap 31a existing between bearing 31 and cylinder block 21. Valve device 193, which has a longitudinally expandable and contractable bellows 193a and valve element 193b attached at a rearward end of bellows 193a, is disposed in valve chamber 192. Bellows 193a longitudinally contracts and expands in response to the crank chamber pressure. Bellows 193a is made of an elastic material, for example, phosphor bronze and has an effective pressure receiving cross-sectional area which is designated below as area A.sub.1. Valve element 193b is generally hemispherical shaped and is attached at the rearward end of bellows 193a. Projection member 193c, which is attached at a forward end of bellows 193a, is secured to axial projection 19c formed at the center of the closed end of casing member 191. Bias spring 193d is longitudinally and compressedly disposed within an inner hollow space of bellows 193a. The resultant force F of the restoring force of bellows 193a and bias spring 193d continuously urges valve element 193b rearwardly (to the right in FIGS. 1 and 2).
Cylinder member 194, which includes valve seat 194a, penetrates the center of valve plate assembly 200, which includes valve plate 25, gaskets 27, 28, suction reed valve 271 and discharge reed valve 281. Valve seat 194a is formed at a forward end of cylinder member 194 and is secured to an opened end of casing member 191. Nut 100 is screwed on cylinder member 194 from a rearward end of cylinder member 194 located in discharge chamber 251 to fix cylinder member 194 to valve plate assembly 200 with valve retainer 253. Conical-shaped opening 194b, which receives valve element 193b, is formed at valve seat 194a and is linked to cylinder 194c axially formed in cylinder member 194. Consequently, annular ridge 194d is formed at a location which is the boundary between conical-shaped opening 194b and cylinder 194c.
When bellows 193a expands to a certain longitudinal length, generally hemispherical-shaped valve element 193b is received by conical-shaped opening 194b to form a circular line contact 193e therebetween. Circular line contact 193e divides valve element 193b into front portion 193f and rear portion 193g, an exterior surface of which is responsive to pressure in suction chamber 241 conducted via later-mentioned radial hole 151, conduit 152 and hole 153. Rear portion 193g of valve element 193b has the effective pressure receiving cross-sectional area which is designated below as area A.sub.2, and which is approximately 50% of the effective pressure receiving cross-sectional area A.sub.1 of bellows 193a.
Actuating rod 195, which is slidably disposed within cylinder 194c, slightly projects from the rearward end of cylinder 194c, and is linked to valve element 193b through bias spring 196, which smoothly transmits the force from actuating rod 195 to valve element 193b of valve device 193. Actuating rod 195 includes annular flange 195a which is integral with and radially extends from an outer surface of a front end portion of actuating rod 195. Annular flange 195a is located in conical shaped opening 194b, and prevents an excessive rearward movement of actuating rod 195 by contacting with annular ridge 194d. O-ring 197 is mounted about actuating rod 195 to seal the mating surfaces of cylinder 194c and actuating rod 195, thereby preventing the invasion of the refrigerant gas from discharge chamber 251 to conical shaped opening 194b via the gap created between cylinder 194c and rod 195. Cup-shaped member 103 having a threaded portion at its inner peripheral side wall is mounted on the rear end portion of cylinder member 194 to prevent O-ring 197 from falling off from the rear end of cylinder member 194.
Radial hole 151 is formed at valve seat 194a to link conical shaped opening 194b to conduit 152 formed in cylinder block 21. Conduit 152, which includes cavity 152a, is linked to suction chamber 241 through hole 153 formed at valve plate assembly 200. Passageway 150, which provides communication between crank chamber 22 and suction chamber 241, includes gap 31a, bore 210, holes 19b, valve chamber 192, conical shaped opening 194b, radial hole 151, conduit 152 and hole 153. As a result, the opening and closing of passageway 150 is controlled by the contraction and expansion of valve device 193 primarily in response to crank chamber pressure.
During operation of compressor 10, drive shaft 26 is rotated by the engine of the vehicle through an electromagnetic clutch 300. Cam rotor 40 is rotated with drive shaft 26, rotating slant plate 50 as well, which causes wobble plate 60 to nutate. Nutational motion of wobble plate 60 reciprocates pistons 71 in their respective cylinders 70. As pistons 71 are reciprocated, refrigerant gas which is introduced into suction chamber 241 through inlet portion 241a flows into each chamber 70 through suction ports 242 and then is compressed. The compressed refrigerant gas is discharged to discharge chamber 251 from each cylinder 70 through discharge ports 252, and therefrom into the cooling circuit through outlet 251a.
The capacity of compressor 10 is adjustable to maintain a constant pressure in suction chamber 241 in response to changes in the heat load on the evaporator or changes in the rotating speed of the compressor. Adjustment of the capacity of the compressor occurs by changing the angle of slant plate 50 which is dependent upon the crank chamber pressure. An increase in crank chamber pressure decreases the slant angle of slant plate 50 and wobble plate 60, decreasing the capacity of the compressor. A decrease in the crank chamber pressure increases the angle of slant plate 50 and wobble plate 60, increasing the capacity of the compressor.
As discussed in U.S. Pat. No. 4,960,367, the effect of valve control mechanism 19' is to maintain a constant pressure at the outlet of the evaporator by controlling the capacity of the compressor in the following manner. Actuating rod 195 pushes valve element 193b in the direction to contract bellows 193a and bias spring 196. Actuating rod 195 moves in response to pressure in discharge chamber 251. Accordingly, increasing pressure in discharge chamber 251 further moves rod 195 toward bellows 193a, thereby increasing the contraction of bellows 193a. As a result, the control point for changing the displacement of the compressor is shifted to maintain a constant pressure at the evaporator outlet. That is, valve control mechanism 19 makes use of the fact that the discharge pressure of the compressor is roughly directly proportional to the suction flow rate. Since actuating rod 195 moves in direct response to changes in discharge pressure, and applies a force directly to valve device 193, the control point at which valve device 193 operates is shifted in a direct and responsive manner by changes in discharge pressure.
Further operation of valve control mechanism 19' is described in detail below. In order to simplify the explanation of the operation of valve control mechanism 19', the above-mentioned effect of valve control mechanism 19' is neglected hereinafter.
With reference to FIGS. 3 and 4, and as particularly illustrated in FIG. 4, in a situation where operation of the compressor is stopped, the suction chamber pressure Ps and the crank chamber pressure Pc are in a state of equilibration, i.e., Pc=Ps, which is greater than the operating point P.sub.1 ' of valve device 193. This causes the contraction of bellows 193a so that valve element 193b permits communication between suction chamber 241 and valve chamber 192 through conical-shaped opening 194b, radial hole 151, conduit 152 and hole 153 to thereby establish communication between crank chamber 22 and suction chamber 241.
In one compressor operational situation indicated by time period "a" in FIG. 4, which is a so-called cool down stage, the compressor operates as follows. In the beginning of operation of the compressor, the communication between crank chamber 22 and suction chamber 241 is maintained, thereby satisfying the equation Pc=Ps as shown by the straight line "l" in FIG. 3 until the suction chamber pressure Ps falls to the operating point P.sub.1 ' of valve device 193. When the suction chamber pressure Ps falls to the operating point P.sub.1 ' of valve device 193, valve element 193b contacts an inner surface of conical-shaped opening 194b due to expansion of bellows 193a. If the suction chamber pressure Ps drops below the operating point P.sub.1 ' of valve device 193, valve element 193b frequently opens and closes conical-shaped opening 194b in accordance with the following equation: EQU F=(A.sub.1 -A.sub.2)Pc+A.sub.2 .multidot.Ps (1)
wherein F is the resultant force of the restoring forces of bellows 193a and bias spring 193d, A.sub.1 is the effective pressure receiving cross-sectional area of bellows 193a, A.sub.2 is the effective pressure receiving cross-sectional area of rear portion 193g of valve element 193b, Ps is the pressure in suction chamber 241, and Pc is the pressure in crank chamber 22. The above equation (1) can be converted into the following equation by solving for Pc: EQU PC=A.sub.2 .multidot.Ps/(A.sub.2 -A.sub.1)+F/(A.sub.1 -A.sub.2)(2)
Equation (2) shows that the crank chamber pressure Pc varies in accordance with the changes in the suction chamber pressure Ps. Furthermore, in this prior art, A.sub.2 is 0.5A.sub.1 so that equation (2) can be further converted to the following equation by substituting 0.5A.sub.1 for A.sub.2. EQU Pc=-Ps+2F/A.sub.1 ( 3)
Equation (3) is shown by the straight line "m" in FIG. 3. Therefore, suction chamber pressure Ps decreases in inverse proportion to the increase in the crank chamber pressure Pc with a proportion of one to one when the suction chamber pressure ps is less than the operating point P.sub.1 ' of valve device 193. At that time, the angular position of slant plate 50 is maintained at the maximum slant angle. However, as illustrated in FIG. 4, once the suction chamber pressure Ps reaches one predetermined pressure P.sub.5 ' at which the pressure difference between the crank and suction chambers 22 and 241 becomes .DELTA.Pmax, the angular position of slant plate 50 shifts to an angle which is smaller than its maximum slant angle. Therefore, the displacement of the compressor shifts to a value which is smaller than the maximum value.
Another compressor operational situation where the heat load on the evaporator gradually decreases is depicted by time period "b" in FIG. 4. As long as the angular position of slant plate 50 is maintained at one angle, suction chamber pressure Ps gradually decreases while the crank chamber pressure Pc gradually increases so as to satisfy equation (3). However, once the suction chamber pressure Ps reaches one predetermined pressure P.sub.5 ' the angular position of slant plate 50 shifts from one angle to another angle which is smaller than the first angle. Therefore, the displacement of the compressor shifts from one value to another value which is smaller than the first value. When the displacement of the compressor shifts to the smaller value due to the change in the angular position of slant plate 50 to a smaller angle, the suction chamber pressure Ps quickly increases because the newly decreased displacement of the compressor insufficiently compensates the heat load on the evaporator. However, this quick increase in the suction chamber pressure Ps hits a peak before the suction chamber pressure Ps reaches another predetermined pressure P.sub.4 ' at which the pressure difference between the crank and suction chambers 22 and 241 becomes .DELTA.Pmin. Thereafter, as long as the angular position of slant plate 50 is maintained at another angle, the suction chamber pressure Ps gradually decreases while the crank chamber pressure Pc gradually increases so as to satisfy equation (3). The above-described operation is repeated while the heat load on the evaporator gradually decreases in accordance with time.
On the other hand, in yet another compressor operation situation where heat load on the evaporator gradually increases in accordance with time, which is indicated by the period "c" in FIG. 4, as long as the angular position of slant plate 50 is maintained at one angle, the suction chamber pressure Ps gradually increases while the crank chamber pressure Pc gradually decreases so as to satisfy equation (3). However, once the suction chamber pressure Ps reaches another predetermined pressure P.sub.4 ', the angular position of slant plate 50 shifts from one angle to another angle which is greater than the first angle. Therefore, the displacement of the compressor shifts from one value to another value which is greater than the first value. When the displacement of the compressor shifts to the greater value due to the change in the angular position of slant plate 50 to a greater angle, the suction chamber pressure Ps quickly decreases because the newly increased displacement of the compressor sufficiently compensates the heat load on the evaporator. However, this quick decrease in the suction chamber pressure Ps bottoms out before the suction chamber pressure Ps reaches one predetermined pressure P.sub.5 '. Thereafter, as long as the angular position of slant plate 50 is maintained at one angle, the suction chamber pressure Ps gradually increases while the crank chamber pressure Pc gradually decreases so as to satisfy equation (3). The above-described operation is repeated while the heat load on the evaporator gradually increases in accordance with time.
Accordingly, during a capacity control stage of operation, which includes time periods "b" and "c" shown in FIG. 4, the suction chamber pressure Ps varies in a range .DELTA.Ps'=P.sub.4 '-P.sub.5 ' while the crank chamber pressure Pc varies in a range .DELTA.Pc'=P.sub.2 '-P.sub.3 ' Furthermore, the range of variation .DELTA.Ps' in the suction chamber pressure is equal to the range of variation .DELTA.Pc' in the crank chamber pressure because the suction chamber pressure Ps decreases in inverse proportion to the increase in the crank chamber pressure Pc at a proportion of one to one. Therefore, the range of variation .DELTA.Ps' in the suction chamber pressure during the capacity control stage is not negligible. Accordingly, when the prior art compressor is used in an automotive air conditioning system, the temperature of cooled air which leaves the evaporator varies over a range which is not negligible so that the air conditioning in a passenger compartment of an automobile is not effectively and efficiently controlled.